The present invention is directed to a station for feeding a web-like matter or sheet, such as a cardboard web, into a station that processes it sequentially, for example a platen press.
Since a platen press is a machine that has to allow a web-like sheet to be temporarily stopped during a cutting operation, an accumulation of the web-like sheet in front of the platen press may occur due to a continuous infeed of the web. Machine designs have included an infeed station which would allow for cyclically monitoring of a formation of a loop of web-like sheet in order to reduce to a maximum the straining effect on this web at the moment when the loop touches a loop monitoring element, owing to the use of a device which carries the web-like sheet around a circumference of an eccentric roller fitted between two rotary plates. A device of this type is described in U.S. Pat. No. 4,060,187, whose disclosure is incorporated herein by reference thereto and which claims priority from a Swiss Application which resulted in Swiss Patent No. 602 462.
Due to the eccentric roller of the loop monitoring device, the constant speed VO of a running web at the inlet of the infeed station is modified with each operating cycle of the machine into an intermittent speed VIT, although the tension of the web is maintained constant. The evolution of the speed VIT throughout the operating cycle of the machine, which cycle is able to be divided into time units, though generally divided into angular position units of the main motor of the station, follows a predetermined and optimized curve with regard to the dynamic characteristics of the web-like sheet.
The web-like sheet, which comes out from the eccentric roller with a modulated speed is driven by a last, so-called infeed roller toward the platen. In order to avoid a collapse of the tension on the web-like sheet at a level of the infeed roller during the maximum running speed VIT, the peripheral speed VRO of this infeed roller is set at a rather high rate, i.e., higher than the maximum speed of the web travelling through the platen press. In other words, the infeed roller is driven permanently by the main motor as well as by a gear-train of the station at a constant rotational speed, which fact makes the peripheral speed of the roller always remain higher than the temporary running speed of the web-like sheet. This action converts into a permanent sliding of the web on the infeed roller, and the action of the frictional force existing between the contact surface of the web and roller is transformed into heat, which fact implies the necessity of an important cooling device for the infeed roller.
For a satisfactory function with standard cardboard, this device quickly reaches its limits when it comes to increasing the production speeds, hence the running speed, particularly with cardboard having a fine layer or coating on the back side, i.e., a polyethylene layer. Then, the heating action created by the frictional forces existing between the infeed roller and the cardboard will heat and partially scrape this fine layer. A plastic material forming the layer becomes pasty, and this tacky plastic material can be sound on the female creasing tools of the platen press. Moreover, inadmissible acceleration marks will appear on the board depending on the position of the infeed belts.
It is almost impossible to increase indefinitely the cooling effect of the water-cooling device of the infeed roller, since increased cooling causes external water condensation which will cause a sliding phenomenon between the web-like sheet and the roller. In addition, the machines may be used in areas which are already overheated, particularly during the summer season.
Thus, the acceleration being proportional to the speed square, it is easily understandable that an increase of 40% in the running speed of the web-like sheet will cause a duplication of the acceleration forces existing which normally have repercussions on the whole of the cinematic general drive chain of the machine, which fact quickly leads to an unrealistic overdimensioning of several of the drive elements.